Rubber composition and golf ball

ABSTRACT

The invention provides a rubber composition having high rebound resilience measured by a rebound resilience evaluation method which is excellent in measurement accuracy and can sufficiently evaluate the difference in performance between samples. The invention also provides a golf ball formed from the rubber composition. The invention further provides a golf ball having excellent flight performance owing to good hardness distribution measured by a hardness evaluation method which is excellent in measurement accuracy and can sufficiently evaluate the difference in performance between samples. The invention relates to a rubber composition having a specific radius of gyration R g  obtained by a predetermined curve fitting method. The invention also relates to a golf ball including a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein the golf ball satisfies a predetermined relation obtained by a predetermined curve fitting method.

TECHNICAL FIELD

The present invention relates to a rubber composition and a golf ball.

BACKGROUND ART

Rubber compositions made from rubber and other materials have been used for various uses such as golf balls. With respect to golf balls, for example, the driver shot distance may be increased by a method of using a highly resilient core, or a method of using a core having hardness distribution such that the hardness increases from the center to the surface of the core. The former method has the effect of increasing the initial speed of a golf ball, whereas the latter method has the effect of increasing launch angle and lowering spin rate. A golf ball with a high launch angle and a low spin can have a longer distance.

Disclosed as a technique for increasing the resilience of a core is a solid golf ball with an inner core which contains rubber together with zinc acrylate as a co-crosslinker, palmitic acid, stearic acid or myristic acid as a co-crosslinking activator, zinc oxide, and a reaction rate retarder (see Patent Literature 1).

Meanwhile, the methods for evaluating the rebound resilience of polymeric materials such as rubber compositions used for golf balls or the like are standardized in JIS K6255 “Testing methods of rebound resilience for rubber, vulcanized or thermoplastic”. In general, a lupke-type rebound resilience test is used in which a pendulum is used to calculate the value from the drop height and the rebound height (see Non-Patent Literature 1).

In a lupke-type rebound resilience tester, a smaller energy loss upon swinging the pendulum leads to greater accuracy of the test. JIS K6255 defines testing methods for determining the period of free oscillation of the pendulum and the logarithmic decrement as a measure of accuracy of measured values.

However, such physical evaluation methods as dropping a pendulum made of, for example, metal onto a polymeric material cause very large errors, and thus the methods are insufficiently satisfactory in measurement accuracy. Another problem is that in the case that the difference in value between samples is small, the difference cannot be evaluated with good reproducibility. In addition, the methods include no method for evaluating the molecular structure in detail.

Thus, it is desired to provide a method for evaluating the resilience (rebound resilience) of rubber compositions, which has excellent measurement accuracy and can evaluate the difference in performance between samples sufficiently, and also provide a rubber composition which has high resilience (rebound resilience) determined by the aforementioned accurate evaluation method and can be suitably used for, for example, cores of golf balls.

Moreover, disclosed as a technique of using a core with hardness distribution is a two-piece golf ball including a core formed from a rubber composition containing a base rubber, a co-crosslinker and an organic peroxide, and a cover, wherein the core has the following hardness distribution: central hardness 1: 58 to 73, hardness 2 within 5 to 10 mm of the center: 65 to 75, hardness 3 within 15 mm of the center: 74 to 82, and surface hardness 4: 76 to 84, each determined by a JIS C-type hardness meter, the hardness 2 is substantially constant within the hardness range, and the hardnesses 1 to 4 satisfy the relation: 1<2<3≦4 (see Patent Literature 2).

Meanwhile, with respect to polymeric materials such as rubber compositions used for golf balls or the like, the hardness is an important physical quantity that affects various properties of products. For example, the hardness of a core of a golf ball (a rubber product) is closely linked to the resilience and spin performances and the like. Widely known as a method for measuring the hardness of rubber products is a method in conformity with JIS K6253 (see Non-Patent Literature 2).

However, measuring methods using a JIS hardness meter cause very large errors, and thus such methods are insufficiently satisfactory in measurement accuracy. Another problem is that in the case that the difference in value between samples is small, the difference cannot be evaluated with good reproducibility. In addition, the methods include no method for evaluating the molecular structure in detail.

Thus, it is desired to provide a method for evaluating the hardness of polymeric materials, which has excellent measurement accuracy and can evaluate the difference in performance between samples sufficiently, and also provide a golf ball which has good hardness distribution determined by the aforementioned accurate evaluation method and is excellent in flight performance.

CITATION LIST Patent Literature

-   Patent Literature 1: JP S61-37178 A -   Patent Literature 2: JP H06-154357 A

Non-Patent Literature

-   Non-Patent Literature 1: JIS K6255 “Testing methods of rebound     resilience for rubber, vulcanized or thermoplastic” -   Non-Patent Literature 2: JIS K6253 “Rubber, vulcanized or     thermoplastic—Determination of hardness”

SUMMARY OF INVENTION Technical Problem

A first aspect of the present invention aims to provide a rubber composition that can solve the problems mentioned above and has high rebound resilience measured by a rebound resilience evaluation method which is excellent in measurement accuracy and capable of sufficiently evaluating the difference in performance between samples; and to provide a golf ball formed from the rubber composition.

A second aspect of the present invention aims to provide a golf ball that can solve the problems mentioned above, has good hardness distribution measured by a hardness evaluation method which is excellent in measurement accuracy and capable of sufficiently evaluating the difference in performance between samples, and is excellent in flight performance.

Solution to Problem

The first aspect of the present invention relates to a rubber composition which has a radius of gyration R_(g) of 5.0 nm or less, the radius of gyration R_(g) being obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 3:

$\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{79mu} {{{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\mspace{70mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)\mspace{70mu} {\left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right).}}} & \left( {{Formula}\mspace{14mu} 3} \right) \end{matrix}$

In the rubber composition, preferably, the X-ray scattering measurement is small-angle X-ray scattering measurement and the neutron scattering measurement is small-angle neutron scattering measurement.

In the rubber composition, the measurement is preferably performed as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹.

The first aspect of the present invention also relates to a golf ball comprising a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from the rubber composition.

In the golf ball, at least one layer of the spherical core is preferably formed from the rubber composition which contains: (a) a base rubber; (b) a co-crosslinker including at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof; (c) a cross-linking initiator; and (d) at least one of an acid and a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound.

In the golf ball, the rubber composition preferably contains 0.5 to 30 parts by mass of the at least one of an acid and a salt thereof (d) per 100 parts by mass of the base rubber (a).

Here, the at least one of an acid and a salt thereof (d) preferably each has 1 to 13 carbon atoms. The at least one of an acid and a salt thereof (d) is preferably at least one of a carboxylic acid and a salt thereof.

In the golf ball, the at least one of a carboxylic acid and a salt thereof is preferably at least one of a fatty acid and a fatty acid salt.

Here, the at least one of a fatty acid and a fatty acid salt is preferably a fatty acid zinc salt.

In the golf ball, the rubber composition preferably further contains (f) an organosulfur compound.

Here, the organosulfur compound (f) is preferably selected from thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.

In the golf ball, the rubber composition preferably contains 0.05 to 5 parts by mass of the organosulfur compound (f) per 100 parts by mass of the base rubber (a).

In the golf ball, the rubber composition preferably contains 15 to 50 parts by mass of the at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof (b) per 100 parts by mass of the base rubber (a).

In the golf ball, the rubber composition preferably contains the metal salt of a C3-C8 α,β-unsaturated carboxylic acid as the co-crosslinker (b).

The second aspect of the present invention relates to a golf ball comprising a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from a rubber composition, and the golf ball satisfies the following relation:

(the number N in an outermost portion of the rubber composition)/(the number N in an innermost portion of the rubber composition)≧2.0

where the number N is the number per unit volume of scatterers with a radius of gyration R_(g) of 1 nm to 100 μm, and the radius of gyration R_(g) is obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 5:

$\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{70mu} {{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}} & \left( {{Formula}\mspace{14mu} 3} \right) \\ {\mspace{70mu} {G_{i} = {N_{i}\left( {\sigma \; V_{i}} \right)}^{2}}} & \left( {{Formula}\mspace{14mu} 4} \right) \\ {\mspace{70mu} {{{V_{i} = {\frac{4}{3}{\pi\left( {\sqrt{\frac{5}{3}}R_{gi}} \right)}^{3}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\left( {N_{i}\text{:}\mspace{14mu} {Number}\mspace{14mu} {of}\mspace{14mu} {scatterers}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {volume}\mspace{14mu} \left( {{pieces}\text{/}{cm}^{3}} \right)} \right)\; \left( {V_{i}\text{:}\mspace{14mu} {Volume}\mspace{14mu} {of}\mspace{14mu} {scatterer}\mspace{14mu} {having}\mspace{14mu} {radius}\mspace{14mu} {of}\mspace{14mu} {gyration}\mspace{14mu} R_{gi}} \right)\mspace{56mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\mspace{56mu} \left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)}\mspace{56mu} \left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right){\left( {\sigma \text{:}\mspace{14mu} {Electron}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {{electron} \cdot {cm}^{- 3}} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {matrix}\mspace{14mu} {material}\mspace{14mu} {or}\mspace{14mu} {scattering}\mspace{14mu} {length}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {cm}^{- 2} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {deuterated}\mspace{14mu} {solvent}} \right).}}} & \left( {{Formula}\mspace{14mu} 5} \right) \end{matrix}$

In the golf ball, preferably, the X-ray scattering measurement is small-angle X-ray scattering measurement and the neutron scattering measurement is small-angle neutron scattering measurement. The measurement is preferably performed as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹.

In the golf ball, the rubber composition preferably contains: (a) a base rubber; (b) a co-crosslinker including at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof; (c) a cross-linking initiator; and (d) at least one of an acid and a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound.

In the golf ball, the rubber composition preferably contains 0.5 to 30 parts by mass of the at least one of an acid and a salt thereof (d) per 100 parts by mass of the base rubber (a).

Here, the at least one of an acid and a salt thereof (d) preferably each has 1 to 13 carbon atoms. The at least one of an acid and a salt thereof (d) is preferably at least one of a carboxylic acid and a salt thereof.

In the golf ball, the at least one of a carboxylic acid and a salt thereof is preferably at least one of a fatty acid and a fatty acid salt.

Here, the at least one of a fatty acid and a fatty acid salt is preferably a fatty acid zinc salt.

In the golf ball, the rubber composition preferably further contains (f) an organosulfur compound.

Here, the organosulfur compound (f) is preferably selected from thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.

In the golf ball, the rubber composition preferably contains 0.05 to 5 parts by mass of the organosulfur compound (f) per 100 parts by mass of the base rubber (a).

In the golf ball, the rubber composition preferably contains 15 to 50 parts by mass of the at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof (b) per 100 parts by mass of the base rubber (a).

In the golf ball, the rubber composition preferably contains the metal salt of a C3-C8 α,β-unsaturated carboxylic acid as the co-crosslinker (b).

Advantageous Effects of Invention

The first aspect of the present invention relates to a rubber composition which has a radius of gyration R_(g) of 5.0 nm or less, wherein the radius of gyration is obtained by curve fitting with predetermined formulas of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement. In other words, the rubber composition has a radius of gyration R_(g) of 5.0 nm or less, where the radius of gyration R_(g) is measured by a rebound resilience evaluation method which is so excellent that it can evaluate rebound resilience with small measurement error and high measurement accuracy, and can accurately evaluate the difference in rebound resilience between different samples between which the difference in performance cannot be evaluated with good reproducibility by a conventional evaluation method. Thus, the rubber composition has very high rebound resilience. Therefore, the rubber composition can be suitably used for cores of golf balls, for example.

The second aspect of the present invention relates to a golf ball comprising a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from a rubber composition, and the golf ball satisfies the following relation: (the number N in an outermost portion of the rubber composition)/(the number N in an innermost portion of the rubber composition)≧2.0, where the number N is the number per unit volume of scatterers with a radius of gyration R_(g) of 1 nm to 100 μm, and the radius of gyration R_(g) is obtained by curve fitting with the formulas (1) to (5) of a scattering intensity curve I_((Q)) obtained by X-ray scattering measurement or neutron scattering measurement. In other words, the golf ball has good hardness distribution measured by a hardness evaluation method which is so excellent that it can evaluate hardness with small measurement error and high measurement accuracy, and can accurately evaluate the difference in hardness between different samples between which the difference in performance cannot be evaluated with good reproducibility by a conventional evaluation method. Therefore, the golf ball of the second aspect of the present invention has a long driver shot distance and excellent flight performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an exemplary scattering intensity curve obtained by SAXS measurement in an embodiment of the first aspect of the present invention.

FIG. 2 shows an exemplary scattering intensity curve obtained by SAXS measurement in an embodiment of the first aspect of the present invention.

FIG. 3 shows an exemplary scattering intensity curve obtained by SAXS measurement in an embodiment of the second aspect of the present invention.

FIG. 4 shows an exemplary scattering intensity curve obtained by SAXS measurement in an embodiment of the second aspect of the present invention.

FIG. 5 shows an exemplary partially cut-away cross-sectional view of a golf ball according to an embodiment of the first and second aspects of the present invention.

DESCRIPTION OF EMBODIMENTS First Aspect of the Present Invention (Rubber Composition)

The rubber composition of the first aspect of the present invention has a radius of gyration R_(g) of 5.0 nm or less, where the radius of gyration R_(g) is obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 3:

$\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{79mu} {{{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\mspace{70mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)\mspace{70mu} {\left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right).}}} & \left( {{Formula}\mspace{14mu} 3} \right) \end{matrix}$

Measurement of small-angle X-ray scattering or small-angle neutron scattering on a rubber composition containing metal atoms or a rubber composition containing filler (e.g. silica) enables to calculate the radius of gyration R_(g) of a cluster formed by aggregation of the metal atoms or filler in the material. The first aspect of the present invention is based on the findings that the radius of gyration R_(g) is highly correlated to rebound resilience and a smaller R_(g) value corresponds to higher rebound resilience. Thus, the X-ray scattering or neutron scattering measurement of the rubber composition enables to evaluate the rebound resilience of the composition, and therefore the rubber composition of the first aspect of the present invention with a radius of gyration R_(g) of not higher than a predetermined value has high rebound resilience.

The reason why the radius of gyration R_(g) correlates to rebound resilience has not clearly been revealed. It is considered that metal atoms are better dispersed as the radius of gyration R_(g) of the cluster is smaller, and consequently, presumably, the energy loss is reduced and in turn the rebound resilience is increased.

(Small-Angle X-Ray Scattering Measurement)

In the first aspect of the present invention, the X-ray scattering measurement in order to evaluate the rebound resilience of the rubber composition may suitably be SAXS (small-angle X-ray scattering (scattering angle: 10 degrees or lower, in general)) measurement in which the rubber composition is irradiated with X-rays so that the scattering intensity is measured. Small-angle X-ray scattering measurement provides information on the structure of a substance by measuring scattered X-rays with small scattering angles among the scattered X-rays resulting from the X-ray exposure to the substance, and it enables to analyze ordered structures of few nanometers, such as a microphase separation structure, of a rubber composition.

In order to obtain detailed information on the molecular structure by SAXS measurement, it is preferable that the X-ray scattering profile with a high S/N ratio be determined. Hence, the X-rays radiated from a synchrotron preferably have a brilliance of at least 10¹⁰ (photons/s/mrad²/mm²/0.1% bw). Here, the symbol bw represents a band width of the X-rays radiated from a synchrotron. Examples of such a synchrotron include the beamlines BL03XU and BL20XU of the large synchrotron radiation facility SPring-8 belonging to Japan Synchrotron Radiation Research Institute.

The brilliance (photons/s/mrad²/mm²/0.1% bw) of the X-rays is preferably 10¹⁰ or higher, and more preferably 10¹²or higher. The upper limit thereof is not particularly limited, and the X-ray intensity used preferably does not reach the minimum intensity which causes radiation damage.

The number of X-ray photons (photons/s) is preferably 10⁷ or more, and more preferably 10⁹ or more. The upper limit is not particularly limited, and the X-ray intensity used preferably does not reach the minimum intensity which causes radiation damage.

(Small-Angle Neutron Scattering Measurement)

Also in the first aspect of the present invention, the neutron scattering measurement in order to evaluate the rebound resilience of the rubber composition may suitably be SANS (small-angle neutron scattering (scattering angle: 10 degrees or lower, in general)) measurement in which the rubber composition is irradiated with a neutron beam so that the scattering intensity is measured. Small-angle neutron scattering measurement provides information on the structure of a substance by measuring scattered neutrons with small scattering angles among the scattered neutrons resulting from the neutron beam exposure to the substance, and it enables to analyze ordered structures of few nanometers, such as a microphase separation structure, of a rubber composition.

The SANS measurement may be performed by a known method utilizing a magnetic structure or a deuteration method. When the deuteration method is used, for example, a rubber composition is swelled by a deuterated solvent, and then the rubber composition equilibrated in the deuterated solvent is irradiated with a neutron beam to measure the scattering intensity. Examples of the deuterated solvent for swelling the rubber composition include heavy water, deuterated hexane, deuterated toluene, deuterated chloroform, deuterated methanol, deuterated DMSO ((D₃C)₂S═O), deuterated tetrahydrofuran, deuterated acetonitrile, deuterated dichloromethane, deuterated benzene, and deuterated N,N-dimethylformamide.

The neutron beam used in the neutron scattering measurement such as SANS may be obtained from the SANS-J beamline of a JRR-3 research reactor belonging to Japan Atomic Energy Agency, an independent administrative agency, for example.

Similarly to the SAXS measurement, in order to obtain a neutron scattering profile with a high S/N ratio, the neutron flux intensity (neutrons/cm²/s) of the neutron beam is preferably 10³ or higher, and more preferably 10⁴ or higher. The upper limit thereof is not particularly limited, and the neutron flux intensity used preferably does not reach the minimum intensity which causes radiation damage.

(Measurement Conditions)

Since the X-ray and neutron scattering measurements are required to measure a finer molecular structure of the rubber composition, each of the measurements is preferably performed using the X-rays or neutron beam as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹. This q range (nm⁻¹) is preferred because a greater numerical value provides finer pieces of information, and the q range is more preferably not greater than 20 nm⁻¹.

The scattered X-rays in the SAXS measurement are detected by an X-ray detector, and then an image is generated by, for example, an image processor based on X-ray detection data from the X-ray detector.

The X-ray detector to be used may be, for example, a two-dimensional detector (e.g. X-ray film, nuclear emulsion plate, X-ray camera tube, X-ray fluorescent amplifier tube, X-ray image intensifier, imaging plate for X-rays, CCD for X-rays, and amorphous material for X-rays), or a line sensor as a one-dimensional detector. The X-ray detector may be selected as appropriate based on the type, state and the like of the rubber composition to be analyzed.

The image processor may be any appropriate one that generates a normal X-ray scattering image based on X-ray detection data from an X-ray detector.

The SANS measurement can be performed under the same principle as the SAXS measurement; in other words, the scattered neutrons are detected by a neutron beam detector, and then an image is generated by a device such as an image processor based on neutron beam detection data from the neutron beam detector. Similarly to the former measurement, the neutron beam detector to be used may be any of known two-dimensional detectors and one-dimensional detectors, and the image processor to be used may be any known one which generates a neutron scattering image. These devices may be selected as appropriate.

(Method of Analyzing Scattering Intensity Curve)

Next, the following will specifically describe the method of analyzing a scattering intensity curve obtained by the X-ray scattering measurement or neutron scattering measurement of a rubber composition.

When the SAXS measurement or the SANS measurement is performed on a rubber composition containing a metal atom and a functional group capable of metal coordination or a rubber composition containing filler to obtain a scattering intensity curve, the radius of gyration (R_(g)) of a cluster (scatterer) with a size of 1 nm to 100 μm may be determined by analyzing the obtained curve by the following method, for example.

Curve fitting of the scattering intensity curve I_((q)) obtained by SAXS measurement as shown in FIG. 1 or the scattering intensity curve I_((q)) obtained by SANS measurement as shown in FIG. 2 is performed with the Formulas 1 to 3, and fitting parameters are determined by a least squares method.

The radius of gyration R_(g) of a molecular structure with a size of 1 nm to 100 μm among the obtained fitting parameters presumably corresponds to the radius of gyration R_(g) of a cluster formed by aggregation of metal atoms or that of a cluster formed by aggregation of filler. Moreover, since the radius of gyration R_(g) is highly correlated to rebound resilience and a smaller R_(g) value corresponds to higher rebound resilience as mentioned above, R_(g) is considered to have a great influence on rebound resilience. Therefore, the rebound resilience of the rubber composition can be evaluated by X-ray scattering measurement (e.g. SAXS) or neutron scattering measurement (e.g. SANS) and then curve fitting with the Formulas 1 to 3 to determine the R_(g) value.

The rubber composition of the first aspect of the present invention has a radius of gyration R_(g) of 5.0 nm or less, where the radius of gyration R_(g) is determined by the aforementioned method for evaluating rebound resilience. Thus, the rubber composition has high rebound resilience (resilience), and can be suitably used for cores of golf balls, for example. The radius of gyration R_(g) is preferably 4.5 nm or less, and more preferably 4.0 nm or less.

(Golf Ball)

Examples of golf balls including a core to which the aforementioned rubber composition is applied according to the first aspect of the present invention include a golf ball including a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from the rubber composition.

The rubber composition having a radius of gyration R_(g) of 5.0 nm or less to be used in at least one layer of the spherical core is not particularly limited as long as the R_(g) value is in the range. For example, it may suitably be a rubber composition which contains: (a) a base rubber; (b) a co-crosslinker including a C3-C8 α,β-unsaturated carboxylic acid and/or a metal salt thereof; (c) a cross-linking initiator; and (d) an acid and/or a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound. Such a composition causes the spherical core to have increased contrast between outer hardness and inner softness, thereby leading to lower spin rate on driver shots and a longer distance.

Second Aspect of the Present Invention (Golf Ball)

The golf ball of the second aspect of the present invention includes a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from a rubber composition, and the golf ball satisfies the relation:

(the number N in an outermost portion of the rubber composition)/(the number N in an innermost portion of the rubber composition)≧2.0

where the number N is the number per unit volume of scatterers with a radius of gyration R_(g) of 1 nm to 100 μm, and the radius of gyration R_(g) is obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 5:

$\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{70mu} {{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}} & \left( {{Formula}\mspace{14mu} 3} \right) \\ {\mspace{70mu} {G_{i} = {N_{i}\left( {\sigma \; V_{i}} \right)}^{2}}} & \left( {{Formula}\mspace{14mu} 4} \right) \\ {\mspace{70mu} {{{V_{i} = {\frac{4}{3}{\pi\left( {\sqrt{\frac{5}{3}}R_{gi}} \right)}^{3}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\left( {N_{i}\text{:}\mspace{14mu} {Number}\mspace{14mu} {of}\mspace{14mu} {scatterers}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {volume}\mspace{14mu} \left( {{pieces}\text{/}{cm}^{3}} \right)} \right)\; \left( {V_{i}\text{:}\mspace{14mu} {Volume}\mspace{14mu} {of}\mspace{14mu} {scatterer}\mspace{14mu} {having}\mspace{14mu} {radius}\mspace{14mu} {of}\mspace{14mu} {gyration}\mspace{14mu} R_{gi}} \right)\mspace{56mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\mspace{56mu} \left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)}\mspace{56mu} \left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right){\left( {\sigma \text{:}\mspace{14mu} {Electron}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {{electron} \cdot {cm}^{- 3}} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {matrix}\mspace{14mu} {material}\mspace{14mu} {or}\mspace{14mu} {scattering}\mspace{14mu} {length}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {cm}^{- 2} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {deuterated}\mspace{14mu} {solvent}} \right).}}} & \left( {{Formula}\mspace{14mu} 5} \right) \end{matrix}$

Measurement of small-angle X-ray scattering or small-angle neutron scattering on a polymeric material such as a metal atom-containing rubber composition enables to calculate the number N of clusters formed by aggregation of metal atoms in the material. The second aspect of the present invention is based on the findings that the number N of clusters is highly correlated to hardness and a greater value N corresponds to higher hardness. Thus, the X-ray scattering or neutron scattering measurement of the polymeric material enables to evaluate the hardness of the material.

The reason why the number N of clusters correlates to hardness has not clearly been revealed. It is considered that the clusters are more densely packed as the number N of clusters increases, and consequently, presumably, the hardness is increased.

(Small-Angle X-Ray Scattering Measurement)

In the second aspect of the present invention, the X-ray scattering measurement in order to evaluate the hardness of the polymeric material may suitably be SAXS (small-angle X-ray scattering (scattering angle: 10 degrees or lower, in general)) measurement in which the polymeric material is irradiated with X-rays so that the scattering intensity is measured. Small-angle X-ray scattering measurement provides information on the structure of a substance by measuring scattered X-rays with small scattering angles among the scattered X-rays resulting from the X-ray exposure to the substance, and it enables to analyze ordered structures of few nanometers, such as a microphase separation structure, of a polymeric material.

In order to obtain detailed information on the molecular structure by SAXS measurement, it is preferable that the X-ray scattering profile with a high S/N ratio be determined. Hence, the X-rays radiated from a synchrotron preferably have a brilliance of at least 10¹⁰ (photons/s/mrad²/mm²/0.1% bw). Here, the symbol bw represents a band width of the X-rays radiated from a synchrotron. Examples of such a synchrotron include the beamlines BL03XU and BL20XU of the large synchrotron radiation facility SPring-8 belonging to Japan Synchrotron Radiation Research Institute.

The brilliance (photons/s/mrad²/mm²/0.1% bw) of the X-rays is preferably 10¹⁰ or higher, and more preferably 10¹² or higher. The upper limit thereof is not particularly limited, and the X-ray intensity used preferably does not reach the minimum intensity which causes radiation damage.

The number of X-ray photons (photons/s) is preferably 10⁷ or more, and more preferably 10⁹ or more. The upper limit is not particularly limited, and the X-ray intensity used preferably does not reach the minimum intensity which causes radiation damage.

(Small-Angle Neutron Scattering Measurement)

Also in the second aspect of the present invention, the neutron scattering measurement in order to evaluate the hardness of the polymeric material may suitably be SANS (small-angle neutron scattering (scattering angle: 10 degrees or lower, in general)) measurement in which the polymeric material is irradiated with a neutron beam so that the scattering intensity is measured. Small-angle neutron scattering measurement provides information on the structure of a substance by measuring scattered neutrons with small scattering angles among the scattered neutrons resulting from the neutron beam exposure to the substance, and it enables to analyze ordered structures of few nanometers, such as a microphase separation structure, of a polymeric material.

The SANS measurement may be performed by a known method utilizing a magnetic structure or a deuteration method. When the deuteration method is used, for example, a polymeric material is swelled by a deuterated solvent, and then the polymeric material equilibrated in the deuterated solvent is irradiated with a neutron beam to measure the scattering intensity. Examples of the deuterated solvent for swelling the polymeric material include heavy water, deuterated hexane, deuterated toluene, deuterated chloroform, deuterated methanol, deuterated DMSO ((D₃C)₂S═O), deuterated tetrahydrofuran, deuterated acetonitrile, deuterated dichloromethane, deuterated benzene, and deuterated N,N-dimethylformamide.

The neutron beam used in the neutron scattering measurement such as SANS may be obtained from the SANS-J beamline of a JRR-3 research reactor belonging to Japan Atomic Energy Agency, an independent administrative agency, for example.

Similarly to the SAXS measurement, in order to obtain a neutron scattering profile with a high S/N ratio, the neutron flux intensity (neutrons/cm²/s) of the neutron beam is preferably 10³ or higher, and more preferably 10⁴ or higher. The upper limit thereof is not particularly limited, and the neutron flux intensity used preferably does not reach the minimum intensity which causes radiation damage.

(Measurement Conditions)

Since the X-ray and neutron scattering measurements are required to measure a finer molecular structure of the polymeric material, each of the measurements is preferably performed using the X-rays or neutron beam as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹. This q range (nm⁻¹) is preferred because a greater numerical value provides finer pieces of information, and the q range is more preferably not greater than 20 nm⁻¹.

The scattered X-rays in the SAXS measurement are detected by an X-ray detector, and then an image is generated by, for example, an image processor based on X-ray detection data from the X-ray detector.

The X-ray detector to be used may be, for example, a two-dimensional detector (X-ray film, nuclear emulsion plate, X-ray camera tube, X-ray fluorescent amplifier tube, X-ray image intensifier, imaging plate for X-rays, CCD for X-rays, and amorphous material for X-rays), or a line sensor as a one-dimensional detector. The X-ray detector may be selected as appropriate based on the type, state and the like of the polymeric material to be analyzed.

The image processor may be any appropriate one that generates a normal X-ray scattering image based on X-ray detection data from an X-ray detector.

The SANS measurement can be performed under the same principle as the SAXS measurement; in other words, the scattered neutrons are detected by a neutron beam detector, and then an image is generated by a device such as an image processor based on neutron beam detection data from the neutron beam detector. Similarly to the former measurement, the neutron beam detector to be used may be any of known two-dimensional detectors and one-dimensional detectors, and the image processor to be used may be any known one which generates a neutron scattering image. These devices may be selected as appropriate.

(Method of Analyzing Scattering Intensity Curve)

Next, the following will specifically describe the method of analyzing a scattering intensity curve obtained by the X-ray scattering measurement or neutron scattering measurement of a polymeric material.

When the SAXS measurement or the SANS measurement is performed on a polymeric material containing a metal atom and a functional group capable of metal coordination to obtain a scattering intensity curve, the number N per unit volume of scatterers with a radius of gyration (R_(g)) of 1 nm to 100 μm may be determined by analyzing the obtained curve by the following method, for example.

Curve fitting of the scattering intensity curve I_((q)) obtained by SAXS measurement as shown in FIG. 1 or the scattering intensity curve I_((q)) obtained by SANS measurement as shown in FIG. 2 is performed with the Formulas 1 to 5, and fitting parameters are determined by a least squares method.

The use of the parameter G among the obtained fitting parameters enables to determine the number N per unit volume of scatterers with a radius of gyration (R_(g)) of 1 nm to 100 μm. The determination of the number N involves the value of the difference in electron density between the scatterer and the surrounding matrix material or the value of the difference in scattering length density. The electron density difference between filler (e.g. silica) and a rubber material (e.g. butadiene rubber), and the scattering length density difference between the scatterer and the surrounding deuterated solvent may each be a known value or a measured value. Specifically, in the case of performing X-ray scattering measurement of a rubber material containing silica as a scatterer (examples of matrix rubber: natural rubber, butadiene rubber, modified butadiene rubber), the value of the electron density difference σ may be 3.8×10²³ (electron·cm⁻³). Also in the case of performing neutron scattering measurement of a polymeric material containing polybutadiene as a scatterer when the material is swelled to equilibrium in deuterated toluene, the value of the scattering length density difference σ may be, for example, 5.22×10¹⁰ (cm⁻²). Moreover, since the number N is highly correlated to hardness and a greater N value corresponds to higher hardness as mentioned above, the number N is considered to have a great influence on hardness. Therefore, the hardness of the polymeric material can be evaluated by X-ray scattering measurement (e.g. SAXS) or neutron scattering measurement (e.g. SANS) and then curve fitting with the Formulas 1 to 5 to determine the number N.

When the symbol N is defined as the number per unit volume of scatterers with a radius of gyration R_(g) of 1 nm to 100 μm, where the radius of gyration R_(g) is obtained by curve fitting with the Formulas 1 to 5 of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement of a rubber composition forming at least one layer of the spherical core, the ratio (the number N in an outermost portion of the rubber composition in the golf ball of the second aspect of the present invention (the number N per unit volume of scatterers with an R_(g) of 1 nm to 100 μm in the outermost portion))/(the number N in an innermost portion of the rubber composition (the number N per unit volume of scatterers with an R_(g) of 1 nm to 100 μm in the innermost portion)) is 2.0 or greater. For example, a single-layer spherical core formed from the rubber composition has a ratio (the number N at the surface portion of the spherical core)/(the number N at the central portion of the spherical core) of 2.0 or greater, whereas a multi-layer spherical core with any layer formed from the rubber composition has a ratio (the number N in the outermost portion of the layer)/(the number N in the innermost portion of the layer) of 2.0 or greater.

Because of such number distribution, or hardness distribution, of scatterers, the golf ball has lower driver spin rate and a longer distance. The ratio (the number N in the outermost portion of the rubber composition)/(the number N in the innermost portion of the rubber composition) is preferably 2.5 or greater, and more preferably 3.0 or greater.

In the golf ball of the second aspect of the present invention, the rubber composition forming at least one layer of the spherical core is not particularly limited as long as it has the aforementioned number distribution. For example, a preferred layer may suitably be formed from a rubber composition which contains: (a) a base rubber; (b) a co-crosslinker including a C3-C8 α,β-unsaturated carboxylic acid and/or a metal salt thereof; (c) a cross-linking initiator; and (d) an acid and/or a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound. Such a composition causes the spherical core to have increased contrast between outer hardness and inner softness, thereby leading to lower spin rate on driver shots and a longer distance.

The following will more specifically describe both the first aspect of the present invention and the second aspect of the present invention.

First, the base rubber (a) is described. The base rubber (a) may be natural rubber and/or synthetic rubber. Examples thereof include polybutadiene rubber, natural rubber, polyisoprene rubber, styrene-polybutadiene rubber, and ethylene-propylene-diene rubber (EPDM). Each of these may be used alone, or two or more of these may be used in combination. Particularly suitable among these is high-cis polybutadiene containing 40% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more, of cis-1,4 bonds which are advantageous to resilience.

The high-cis polybutadiene preferably contains 2% by mass or less, more preferably 1.7% by mass or less, and still more preferably 1.5% by mass or less, of 1,2-vinyl bonds. Too high a 1,2-vinyl bond content may cause reduced resilience.

The high-cis polybutadiene is preferably synthesized in the presence of a rare-earth catalyst. In particular, neodymium catalysts containing a neodymium compound which is a lanthanide rare-earth element compound are preferred because such catalysts allow the production of polybutadiene rubber with a high 1,4-cis bond content and a low 1,2-vinyl bond content at an excellent polymerization activity.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 or higher, more preferably 32 or higher, and still more preferably 35 or higher, whereas the Mooney viscosity is preferably 140 or lower, more preferably 120 or lower, still more preferably 100 or lower, and most preferably 80 or lower. The Mooney viscosity (ML₁₊₄(100° C.)) in the present invention is a value measured using an L rotor at a preheating time of one minute, a rotation time of the rotor of four minutes, and a temperature of 100° C. in accordance with JIS K6300.

The high-cis polybutadiene preferably has a molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of 2.0 or greater, more preferably 2.2 or greater, still more preferably 2.4 or greater, and most preferably 2.6 or greater, whereas Mw/Mn is preferably 6.0 or smaller, more preferably 5.0 or smaller, still more preferably 4.0 or smaller, and most preferably 3.4 or smaller. Too low a molecular weight distribution (Mw/Mn) of the high-cis polybutadiene may cause reduced workability, whereas too high a molecular weight distribution may cause reduced resilience. The molecular weight distribution is a value measured by gel permeation chromatography (HLC-8120GPC, TOSOH CORP.) with a differential refractometer as the detector (column: GMHHXL (TOSOH CORP.), column temperature: 40° C., mobile phase: tetrahydrofuran) and calculated relative to polystyrene standards.

Next, the C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b) is described. The C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b) is incorporated as a co-crosslinker into the rubber composition, and it is graft-polymerized with the molecular chain of the base rubber to serve to cross-link the rubber molecules. When the co-crosslinker in the rubber composition used in the present invention only includes the C3-C8 α,β-unsaturated carboxylic acid, the rubber composition further contains (e) a metal compound as an essential component. This is because the metal compound neutralizes the C3-C8 α,β-unsaturated carboxylic acid in the rubber composition, so that substantially the same effects can be obtained as when a metal salt of a C3-C8 α,β-unsaturated carboxylic acid is used as the co-crosslinker. Also in the case that a C3-C8 α,β-unsaturated carboxylic acid and a salt thereof are used in combination as the co-crosslinker, the rubber composition may contain the metal compound (e) as an optional component.

Examples of the C3-C8 α,β-unsaturated carboxylic acid include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid.

Examples of metals that can form the metal salt of C3-C8 α,β-unsaturated carboxylic acid include: monovalent metal ions such as sodium, potassium, and lithium; divalent metal ions such as magnesium, calcium, zinc, barium, and cadmium; trivalent metal ions such as aluminum; and other metal ions such as tin and zirconium. Each of these metal components may be used alone or two or more of these may be used in admixture. Preferred metal components among these are divalent metals such as magnesium, calcium, zinc, barium, and cadmium. This is because the use of a divalent metal salt of the C3-C8 α,β-unsaturated carboxylic acid allows easy formation of metal crosslinks between the rubber molecules. Particularly preferred among the divalent metal salts is zinc acrylate because it contributes to a highly resilient golf ball. Each of the C3-C8 α,β-unsaturated carboxylic acids and/or metal salts thereof may be used alone, or two or more of these may be used in combination.

The amount of the C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b) is preferably 15 parts by mass or more, more preferably 20 parts by mass or more, whereas it is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, and still more preferably 35 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount of the C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b) is less than 15 parts by mass, an increased amount of the cross-linking initiator (c) as described later is required in order to provide an appropriate hardness to a member formed from the rubber composition, and thus the resilience of the resulting golf ball tends to be reduced. Conversely, if the amount of the C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b) exceeds 50 parts by mass, a member formed from the rubber composition may be so hard that the resulting golf ball may deteriorate in terms of shot feeling.

The cross-linking initiator (c) is incorporated for the purpose of cross-linking the base rubber (a). The cross-linking initiator (c) may suitably be an organic peroxide. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Each of these organic peroxides may be used alone, or two or more of these may be used in combination. Preferably used among these is dicumyl peroxide.

The amount of the cross-linking initiator (c) is preferably 0.2 parts by mass or more, more preferably 0.5 parts by mass or more, whereas it is preferably 5.0 parts by mass or less, and more preferably 2.5 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount is less than 0.2 parts by mass, a member formed from the rubber composition may be so soft that the resilience of the resulting golf ball tends to be reduced. If the amount exceeds 5.0 parts by mass, the amount of the co-crosslinker (b) described above needs to be reduced in order to provide an appropriate hardness to a member formed from the rubber composition, and thus the resilience of the golf ball may be insufficient and the durability may be poor.

Next, the acid and/or salt thereof (d) is described. Examples of the acid component in the acid and/or salt thereof (d) used in the present invention include: oxoacids such as carboxylic acids, sulfonic acids, and phosphoric acids; and hydroacids such as hydrochloric acid and hydrofluoric acid. Preferred among these are oxoacids, and more preferred are carboxylic acids. The component (d) does not encompass C3-C8 α,β-unsaturated carboxylic acids and salts thereof to be used as the co-crosslinker (b). The acid and/or salt thereof (d) is considered to have the effect of cleaving metal crosslinks formed by the metal salt of C3-C8 α,β-unsaturated carboxylic acid (b), at the central portion of the core during the formation of the core.

As the core hardness distribution shows substantially linear increases from the core center towards the surface and the core has increased contrast between outer hardness and inner softness, the golf ball has lower driver spin rate and a longer distance. The reason why the core hardness distribution shows substantially linear increases from the core center towards the surface is presumably as follows. With respect to the temperature inside the core during the formation of the core, it is high at the central portion of the core and decreases towards the core surface because the reaction heat of cross-linking reaction remains in the core central portion. The acid and/or salt thereof (d) reacts with the metal salt of C3-C8 α,β-unsaturated carboxylic acid (b) to form a metal salt of carboxylic acid during the core formation. This means that the acid and/or salt thereof (d) and the metal salt of C3-C8 α,β-unsaturated carboxylic acid exchange their cations, so that the metal crosslinks formed by the metal salt of C3-C8 α,β-unsaturated carboxylic acid are cleaved. This cation-exchange reaction is more likely to occur in the core central portion where the temperature is higher, whereas it is less likely to occur towards the surface. In other words, the cleavage of metal crosslinks is more likely to occur in the core central portion and is less likely to occur towards the surface. As a result, the crosslink density inside the core increases from the core center towards the surface, and it is therefore considered that the core hardness substantially linearly increases from the core center towards the surface.

Further studies demonstrate that the use of the acid and/or salt thereof (d) causes the spherical core to have increased contrast between outer hardness and inner softness, thereby leading to lower spin rate on driver shots. The reason why the use of the acid and/or salt thereof (d) causes the spherical core to have increased contrast between outer hardness and inner softness is presumably as follows. The acid and/or salt thereof and the metal salt of C3-C8 α,β-unsaturated carboxylic acid exchange their cations, so that the metal crosslinks formed by the metal salt of C3-C8 α,β-unsaturated carboxylic acid are cleaved. This effect of cleaving metal crosslinks is considered to be affected by the amount in moles of the acid and/or salt thereof to be added. At the same time, the acid and/or salt thereof serves as a plasticizer for the spherical core. As the amount (mass) of the acid and/or salt thereof added increases, the whole core becomes softer. Such a plasticizing effect is affected by the amount (mass) of the acid and/or salt thereof to be added. Considering these effects, when the acid and/or salt thereof with a smaller number of carbon atoms (a smaller molecular weight) is used in the same amount (in mass), the amount in moles added is greater than when the acid and/or salt thereof with a larger number of carbon atoms (a larger molecular weight) is used. In other words, the effect of cleaving metal crosslinks can be enhanced while the plasticizing effect suppresses softening of the whole spherical core.

The acid and/or salt thereof (d) may be any of an aliphatic acid and/or a salt thereof (also referred to simply as “a fatty acid and/or a fatty acid salt” in the present invention) and an aromatic acid and/or a salt thereof as long as they can exchange cation components with the metal salt of C3-C8 α,β-unsaturated carboxylic acid. Those with a small number of carbon atoms have some problems such as toxicity and an offensive smell. The number of carbon atoms here is the number of carbon atoms in the acid component.

The fatty acid and/or fatty acid salt may be any of a saturated fatty acid and/or a saturated fatty acid salt and an unsaturated fatty acid and/or an unsaturated fatty acid salt, and it is preferably a saturated fatty acid and/or a saturated fatty acid salt. Specific examples of the fatty acid component in the fatty acid and/or fatty acid salt include butyric acid (C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylic acid (octanoic acid) (C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), and lauric acid (C12). Each of these acids as the fatty acid component may be used alone, or two or more of these may be used in admixture. Preferred as the fatty acid component are caprylic acid (octanoic acid) (C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), and lauric acid (C12). More preferred are caprylic acid (octanoic acid) (C8), capric acid (decanoic acid) (C10), and lauric acid (C12).

Specific examples of the aromatic acid component in the aromatic acid and/or salt thereof include benzoic acid (C7), phthalic acid (C8), isophthalic acid (C8), terephthalic acid (C8), hemimellitic acid (benzene-1,2,3-tricarboxylic acid) (C9), trimellitic acid (benzene-1,2,4-tricarboxylic acid) (C9), trimesic acid (benzene-1,3,5-tricarboxylic acid) (C9), mellophanic acid (benzene-1,2,3,4-tetracarboxylic acid) (C10), prehnitic acid (benzene-1,2,3,5-tetracarboxylic acid) (C10), pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid) (C10), mellitic acid (benzenehexacarboxylic acid) (C12), diphenic acid (biphenyl-2,2′-dicarboxylic acid) (C12), toluic acid (methylbenzoic acid) (C8), xylylic acid (C9), prehnitylic acid (2,3,4-trimethylbenzoic acid) (C10), γ-isodurylic acid (2,3,5-trimethylbenzoic acid) (C10), durylic acid (2,4,5-trimethylbenzoic acid) (C10), β-isodurylic acid (2,4,6-trimethylbenzoic acid) (C10), α-isodurylic acid (3,4,5-trimethylbenzoic acid) (C10), cuminic acid (4-isopropylbenzoic acid) (C10), uvitic acid (5-methylisophthalic acid) (C9), α-toluic acid (phenylacetic acid) (C8), hydratropic acid (2-phenylpropanoic acid) (C9), and hydrocinnamic acid (3-phenylpropanoic acid) (C9).

Examples of the aromatic acid component substituted with a hydroxyl group, an alkoxy group, or an oxo group include salicylic acid (2-hydroxybenzoic acid) (C7), anisic acid (methoxybenzoic acid) (C8), cresotinic acid (hydroxy(methyl)benzoic acid) (C8), o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid) (C8), m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid) (C8), p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid) (C8), o-pyrocatechuic acid (2,3-dihydroxybenzoic acid) (C7), β-resorcylic acid (2,4-dihydroxybenzoic acid) (C7), γ-resorcylic acid (2,6-dihydroxybenzoic acid) (C7), protocatechuic acid (3,4-dihydroxybenzoic acid) (C7), α-resorcylic acid (3,5-dihydroxybenzoic acid) (C7), vanillic acid (4-hydroxy-3-methoxybenzoic acid) (C8), isovanillic acid (3-hydroxy-4-methoxybenzoic acid) (C8), veratric acid (3,4-dimethoxybenzoic acid) (C9), o-veratric acid (2,3-dimethoxybenzoic acid) (C9), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid) (C8), m-hemipinic acid (4,5-dimethoxyphthalic acid) (C10), gallic acid (3,4,5-trihydroxybenzoic acid) (C7), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid) (C9), asaronic acid (2,4,5-trimethoxybenzoic acid) (C10), mandelic acid (hydroxy(phenyl)acetic acid) (C8), vanillylmandelic acid (hydroxy (4-hydroxy-3-methoxyphenyl)acetic acid) (C9), homoanisic acid ((4-methoxyphenyl)acetic acid) (C9), homogentisic acid ((2,5-dihydroxyphenyl)acetic acid) (C8), homoprotocatechuic acid ((3,4-dihydroxyphenyl)acetic acid) (C8), homovanillic acid ((4-hydroxy-3-methoxyphenyl)acetic acid) (C9), homoisovanillic acid ((3-hydroxy-4-methoxyphenyl)acetic acid) (C9), homoveratric acid ((3,4-dimethoxyphenyl)acetic acid) (C10), o-homoveratric acid ((2,3-dimethoxyphenyl)acetic acid) (C10), homophthalic acid (2-(carboxymethyl)benzoic acid) (C9), homoisophthalic acid (3-(carboxymethyl)benzoic acid) (C9), homoterephthalic acid (4-(carboxymethyl)benzoic acid) (C9), phthalonic acid (2-(carboxycarbonyl)benzoic acid) (C9), isophthalonic acid (3-(carboxycarbonyl)benzoic acid) (C9), terephthalonic acid (4-(carboxycarbonyl)benzoic acid) (C9), atrolactic acid (2-hydroxy-2-phenylpropanoic acid) (C9), tropic acid (3-hydroxy-2-phenylpropanoic acid) (C9), melilotic acid (3-(2-hydroxyphenyl)propanoic acid) (C9), phloretic acid (3-(4-hydroxyphenyl)propanoic acid) (C9), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid) (C9), hydroferulic acid (3-(4-hydroxy-3-methoxyphenyl)propanoic acid) (C10), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl)propanoic acid) (C10), p-coumaric acid (3-(4-hydroxyphenyl)acrylic acid) (C9), umbellic acid (3-(2,4-dihydroxyphenyl)acrylic acid) (C9), caffeic acid (3-(3,4-dihydroxyphenyl)acrylic acid) (C9), ferulic acid (3-(4-hydroxy-3-methoxyphenyl)acrylic acid) (C10), isoferulic acid (3-(3-hydroxy-4-methoxyphenyl)acrylic acid) (C10), and sinapinic acid (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid) (C11).

The cation component in the acid salt may either be a metal ion or an organic cation. Examples of the metal ion include: monovalent metal ions such as sodium, potassium, lithium, and silver; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel, and manganese; trivalent metal ions such as aluminum and iron; and other metal ions such as tin, zirconium, and titanium. The cation component in the acid salt is preferably a zinc ion. Each of these cations as the cation component may be used alone, or two or more of these may be used in admixture.

The organic cation refers to a cation having a carbon chain. The organic cation is not particularly limited, and examples thereof include organic ammonium ions. Examples of the organic ammonium ions include: primary ammonium ions such as a stearyl ammonium ion, a hexyl ammonium ion, an octyl ammonium ion, and a 2-ethylhexyl ammonium ion; secondary ammonium ions such as a dodecyl(lauryl) ammonium ion and an octadecyl(stearyl) ammonium ion; tertiary ammonium ions such as a trioctyl ammonium ion; and quaternary ammonium ions such as a dioctyldimethyl ammonium ion and a distearyldimethyl ammonium ion. Each of these organic cations may be used alone, or two or more of these may be used in combination.

The acid and/or salt thereof (d) is preferably a carboxylic acid and/or a salt thereof, and the carbon number thereof is preferably 1 to 13. Fatty acid zinc salts are also preferred. Preferred among these are metal salts of caprylic acid (octanoic acid) (C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), or lauric acid (C12), and more preferred are zinc salts of caprylic acid (octanoic acid) (C8), capric acid (decanoic acid) (C10), or lauric acid (C12). The carbon number of a C1 to C13 carboxylic acid salt means the carbon number of the carboxylic acid component. More specifically, in the case of an organic cation salt of carboxylic acid, for example, the carbon atoms in the organic cation are not included in the carbon number.

The amount of the acid and/or salt thereof (d) is preferably 0.5 parts by mass or more, more preferably 1.0 part by mass or more, and still more preferably 1.5 parts by mass or more, whereas it is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and still more preferably 20 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount is too small, the added acid and/or salt thereof (d) may insufficiently exert its effects, so that the spherical core may have reduced contrast between outer hardness and inner softness. If the amount is too large, the hardness of the core to be obtained may be low as a whole and the resilience may be reduced. Meanwhile, the surface of zinc acrylate to be used as the co-crosslinker may be treated with the acid and/or salt thereof (d) in order to improve its dispersibility into rubber. In the present invention, in the case of using zinc acrylate that is surface-treated with the acid and/or salt thereof (d), the amount of the acid and/or salt thereof (d) used as the surface-treating agent is not included in the amount of the component (d). Such acid and/or salt thereof (d) used for surface-treating zinc acrylate is considered to hardly contribute to the cation exchange reaction with the co-crosslinker (b).

When the co-crosslinker in the rubber composition used in the present invention only includes the C3-C8 α,β-unsaturated carboxylic acid, the rubber composition further contains (e) a metal compound as an essential component. The metal compound (e) is not particularly limited as long as it neutralizes the C3-C8 α,β-unsaturated carboxylic acid (b) in the rubber composition. Examples of the metal compound (e) include: metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. Preferred as the metal compound (e) are divalent metal compounds, and more preferred are zinc compounds. This is because divalent metal compounds react with the C3-C8 α,β-unsaturated carboxylic acid to form metal crosslinks. Moreover, zinc compounds contribute to a highly resilient golf ball. Each of these metal compounds (e) may be used alone, or two or more of these may be used in combination. The amount of the metal compound (e) may appropriately be adjusted according to the amount of the C3-C8 α,β-unsaturated carboxylic acid (b).

The rubber composition for a core in the present invention preferably further contains (f) an organosulfur compound. When the rubber composition for a core contains the organosulfur compound (f) in addition to the C1-C13 carboxylic acid salt (d), it is possible to adjust the contrast between outer hardness and inner softness inside the core while maintaining a substantially linear core hardness distribution. The organosulfur compound (f) is not particularly limited as long as it is an organic compound containing a sulfur atom in its molecule.

Examples thereof include organic compounds containing a thiol group (—SH) or a polysulfide bond containing 2 to 4 sulfur atoms (—S—S—, —S—S—S—, or —S—S—S—S—), and metal salts thereof (e.g. —SM, —S-M-S—, —S-M-S—S—, —S—S-M-S—S—, —S-M-S—S—S— where M represents a metal atom). The organosulfur compound (f) may be any of aliphatic compounds (e.g. aliphatic thiols, aliphatic thiocarboxylic acids, aliphatic dithiocarboxylic acids, aliphatic polysulfides), heterocyclic compounds, alicyclic compounds (e.g. alicyclic thiols, alicyclic thiocarboxylic acids, alicyclic dithiocarboxylic acids, alicyclic polysulfides), and aromatic compounds. Examples of the organosulfur compound (f) include thiophenols, thionaphthols, polysulfides (e.g. diphenyl disulfides), thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams (e.g. thiuram disulfides), dithiocarbamic acid salts, and thiazoles. From the viewpoint of broader hardness distribution of the spherical core, the organosulfur compound (f) is preferably an organosulfur compound containing a thiol group (—SH) or a metal salt thereof, preferably a thiophenol, a thionaphthol, or a metal salt thereof. Examples of the metal salts include: salts with monovalent metals such as sodium, lithium, potassium, copper (I), and silver (I); and salts with divalent metals such as zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), and tin (II).

Examples of the thiophenols include thiophenol; thiophenols substituted with a fluoro group such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, and pentafluorothiophenol; thiophenols substituted with a chloro group such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, and pentachlorothiophenol; thiophenols substituted with a bromo group such as 4-bromothiophenol, 2,5-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, and pentabromothiophenol; thiophenols substituted with an iodo group such as 4-iodothiophenol, 2,5-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; and metal salts thereof. Preferred metal salts are zinc salts.

Examples of the thionaphthols include 2-thionaphthol, 1-thionaphthol, 2-chloro-1-thionaphthol, 2-bromo-1-thionaphthol, 2-fluoro-1-thionaphthol, 2-cyano-1-thionaphthol, 2-acetyl-1-thionaphthol, 1-chloro-2-thionaphthol, 1-bromo-2-thionaphthol, 1-fluoro-2-thionaphthol, 1-cyano-2-thionaphthol, and 1-acetyl-2-thionaphthol, and metal salts thereof. Preferred among these are 1-thionaphthol, 2-thionaphthol, and zinc salts thereof.

Examples of the sulfenamide-based organosulfur compounds include N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazolesulfenamide, and N-t-butyl-2-benzothiazolesulfenamide. Examples of the thiuram-based organosulfur compounds include tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. Examples of the dithiocarbamic acid salts include zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zinc ethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III) dimethyldithiocarbamate, selenium diethyldithiocarbamate, and tellurium diethyldithiocarbamate. Examples of the thiazole-based organosulfur compounds include 2-mercaptobenzothiazole (MBT), dibenzothiazyl disulfide (MBTS), sodium, zinc, copper or cyclohexylamine salts of 2-mercaptobenzothiazole, 2-(2,4-dinitrophenyl)mercaptobenzothiazole, and 2-(2,6-diethyl-4-morpholinothio)benzothiazole.

The organosulfur compound (f) may be one of the above compounds or a mixture of two or more of the above compounds.

The amount of the organosulfur compound (f) is preferably 0.05 parts by mass or more, and more preferably 0.1 parts by mass or more, whereas it is preferably 5.0 parts by mass or less, and more preferably 2.0 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount is less than 0.05 parts by mass, the added organosulfur compound (f) may not exert its effects, so that the resilience of the golf ball may not be enhanced. If the amount exceeds 5.0 parts by mass, the compression deformation of the golf ball to be obtained may be large and the resilience may be reduced.

The rubber composition used in the present invention may contain any additives such as pigments, filler for weight adjustment and the like, antioxidants, peptizers, and softening agents, as appropriate. Further, as mentioned above, preferably, when the co-crosslinker in the rubber composition in the present invention only includes the C3-C8 α,β-unsaturated carboxylic acid, the rubber composition further contains the metal compound (e).

Examples of the pigments to be incorporated into the rubber composition include white pigments, blue pigments, and purple pigments. Preferred white pigments include titanium oxide. The type of titanium oxide is not particularly limited, and is preferably rutile titanium oxide because it provides good hiding properties. The amount of titanium oxide is preferably 0.5 parts by mass or more, and more preferably 2 parts by mass or more, whereas it is preferably 8 parts by mass or less, and more preferably 5 parts by mass or less, per 100 parts by mass of the base rubber (a).

In one preferred embodiment, the rubber composition contains both a white pigment and a blue pigment. The blue pigment is incorporated for the purpose of brightening the white color. Examples thereof include ultramarine, cobalt blue, and phthalocyanine blue. Examples of the purple pigments include anthraquinone violet, dioxazine violet, and methyl violet.

The amount of the blue pigment is preferably 0.001 parts by mass or more, and more preferably 0.05 parts by mass or more, whereas it is preferably 0.2 parts by mass or less, and more preferably 0.1 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount is less than 0.001 parts by mass, the blueness is insufficient and the color looks yellowish. If the amount exceeds 0.2 parts by mass, the blueness is so strong that the vivid white appearance is lost.

The filler to be used in the rubber composition is intended to be incorporated mainly as a weighting agent for adjusting the weight of a golf ball obtained as the final product, and may be incorporated as appropriate. Examples of the filler include inorganic filler such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, and molybdenum powder. The amount of the filler is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, whereas it is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and still more preferably 20 parts by mass or less, per 100 parts by mass of the base rubber (a). If the amount of the filler is less than 0.5 parts by mass, the filler is less likely to adjust the weight. If the amount exceeds 30 parts by mass, the weight fraction of the rubber component is reduced and the resilience tends to be reduced.

The amount of the antioxidant is preferably at least 0.1 parts by mass but not more than 1 part by mass per 100 parts by mass of the base rubber (a). The amount of the peptizer is preferably at least 0.1 parts by mass but not more than 5 parts by mass per 100 parts by mass of the base rubber (a).

The rubber composition used in the present invention can be obtained by mixing and kneading the base rubber (a), the C3-C8 α,β-unsaturated carboxylic acid and/or metal salt thereof (b), the cross-linking initiator (c), the acid and/or salt thereof (d), and optionally any other additives as appropriate. The kneading method is not particularly limited, and kneading may be performed using a known kneading machine such as a kneading roll, a Banbury mixer, or a kneader.

The spherical core in the golf ball of the present invention can be obtained by molding the kneaded rubber composition in a mold. The temperature for molding the rubber composition into a spherical core is preferably 120° C. or higher, more preferably 150° C. or higher, and still more preferably 160° C. or higher, whereas it is preferably 170° C. or lower. The molding temperature higher than 170° C. tends to cause reduction in core surface hardness. The pressure during the molding is preferably 2.9 to 11.8 MPa. The molding time is preferably 10 to 60 minutes.

The spherical core preferably has an R² value of a linear approximation curve of 0.95 or higher, where the linear approximation curve is determined by plotting hardnesses measured at nine points obtained by dividing a radius of the core into equal parts at intervals of 12.5% against the corresponding distances from the core center, and then applying a least squares method. The R² value of 0.95 or higher corresponds to the core with a more linear hardness distribution leading to lower driver spin rate and a longer distance.

With respect to the hardness of the spherical core, the JIS-C hardness is measured at nine points obtained by dividing an arbitrary radius of the spherical core into equal parts at intervals of 12.5%. In other words, the JIS-C hardness is measured at nine points at distances from the core center of 0% (core center), 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, 87.5%, and 100% (core surface). Next, the measurement results are plotted to draw a graph where the vertical axis shows the JIS-C hardness measured above and the horizontal axis shows the distance (%) from the core center. In the present invention, the R² value of a linear approximation curve obtained from the plots by a least squares method is preferably 0.95 or higher. The R² value of a linear approximation curve obtained by a least squares method is an indicator of linearity of the obtained plots. In the present invention, the R² value of 0.95 or higher indicates that the hardness distribution of the spherical core is substantially linear. A golf ball including a spherical core with a substantially linear hardness distribution has lower spin rate on driver shots. Consequently, it has a longer driver shot distance. The R² value of the linear approximation curve is more preferably 0.96 or higher. As the linearity increases, the driver shot distance increases.

The difference in hardness (Hs—Ho) between the surface hardness Hs and the central hardness Ho of the spherical core is, in terms of JIS-C hardness, preferably 27 or greater, more preferably 28 or greater, and still more preferably 30 or greater, whereas it is preferably 80 or smaller, more preferably 70 or smaller, and still more preferably 60 or smaller. A greater difference in hardness between the core surface and the core center leads to a golf ball with a higher launch angle, lower spin, and a longer distance.

The central hardness Ho of the spherical core is, in terms of JIS-C hardness, preferably 30 or higher, more preferably 40 or higher, and still more preferably 45 or higher. If the central hardness Ho of the spherical core is lower than 30 in terms of JIS-C hardness, the core may be so soft that the resilience may be reduced. The central hardness Ho of the spherical core is, in terms of JIS-C hardness, preferably 70 or lower, more preferably 65 or lower, and still more preferably 60 or lower. If the central hardness Ho exceeds 70 in terms of JIS-C hardness, the core tends to be so hard that the resulting golf ball may deteriorate in terms of shot feeling.

The surface hardness Hs of the spherical core is, in terms of JIS-C hardness, preferably 76 or higher, and more preferably 78 or higher, whereas it is preferably 100 or lower, and more preferably 95 or lower. The spherical core having a surface hardness of 76 or higher in terms of JIS-C hardness has moderate softness and offers favorable resilience. Also, the spherical core having a surface hardness of 100 or lower in terms of JIS-C hardness has moderate hardness and offers favorable shot feeling.

The diameter of the spherical core is preferably 34.8 mm or greater, more preferably 36.8 mm or greater, and still more preferably 38.8 mm or greater, whereas it is preferably 42.2 mm or smaller, more preferably 41.8 mm or smaller, still more preferably 41.2 mm or smaller, and most preferably 40.8 mm or smaller. The spherical core having a diameter of 34.8 mm or greater allows a cover to have moderate thickness, thereby leading to more favorable resilience. Also, the spherical core having a diameter of 42.2 mm or smaller allows a cover to have moderate thinness and therefore to further exert its effects.

The spherical core with a diameter of 34.8 to 42.2 mm preferably has a compression deformation (amount of shrinkage of the core in the compression direction) measured from an initial load of 98 N to a final load of 1275 N of 2.0 mm or greater, and more preferably 2.8 mm or greater, whereas the compression deformation is preferably 6.0 mm or smaller, and more preferably 5.0 mm or smaller. The compression deformation of 2.0 mm or greater leads to better shot feeling. The compression deformation of 6.0 mm or smaller leads to better resilience.

The cover of the golf ball of the present invention is formed from a composition for a cover containing a resin component. Examples of the resin component include ionomer resins, thermoplastic polyurethane elastomers marketed by BASF Japan Ltd. under the trade name “Elastollan (registered trademark)”, thermoplastic polyamide elastomers marketed by Arkema Inc. under the trade name “Pebax (registered trademark)”, thermoplastic polyester elastomers marketed by DU PONT-TORAY CO., LTD. under the trade name “Hytrel (registered trademark)”, and thermoplastic styrene elastomers marketed by Mitsubishi Chemical Corp. under the trade name “RABALON (registered trademark)”.

Examples of the ionomer resins include: metal ion-neutralized products of a binary copolymer of an olefin and a C3-C8 α,β-unsaturated carboxylic acid, in which at least part of carboxyl groups in the copolymer are neutralized by a metal ion; metal ion-neutralized products of a ternary copolymer of an olefin, a C3-C8 α,β-unsaturated carboxylic acid and an α,β-unsaturated carboxylic acid ester, in which at least part of carboxyl groups are neutralized by a metal ion; and mixtures thereof. The olefin is preferably a C2-C8 olefin, and examples thereof include ethylene, propylene, butene, pentene, hexene, heptene, and octene. Particularly preferred is ethylene. Examples of the C3-C8 α,β-unsaturated carboxylic acid include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid, and particularly preferred are acrylic acid and methacrylic acid. Examples of the α,β-unsaturated carboxylic acid ester include methyl, ethyl, propyl, n-butyl or isobutyl esters of acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like, and particularly preferred are acrylic acid esters and methacrylic acid esters. Among these, preferred ionomer resins are metal ion-neutralized products of ethylene/(meth)acrylic acid binary copolymers and metal ion-neutralized products of ethylene/(meth)acrylic acid/(meth)acrylic acid ester ternary copolymers.

Specific examples of the ionomer resins in terms of trade name include “Himilan (registered trademark) (e.g. Himilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na), and Himilan AM3711 (Mg); and the terpolymer ionomer resins Himilan 1856 (Na) and Himilan 1855 (Zn))” marketed by DUPONT-MITSUI POLYCHEMICALS CO., LTD.

Further, DU PONT markets ionomer resins under the trade name “Surlyn (registered trademark) (e.g. Surlyn 8945 (Na), Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li), and Surlyn AD8546 (Li); and the terpolymer ionomer resins Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), F1000 (Mg), and HPF2000 (Mg))”.

Moreover, ExxonMobil Chemical markets ionomer resins under the trade name “Iotek (registered trademark) (e.g. Iotek 8000 (Na), Iotek 8030 (Na), Iotek 7010 (Zn), and Iotek 7030 (Zn); and the terpolymer ionomer resins Iotek 7510 (Zn) and Iotek 7520 (Zn))”.

The symbols such as Na, Zn, Li, and Mg put in the parentheses which follow the trade names of the ionomer resins indicate metal species of metal ions used for neutralization of these resins. Each of the ionomer resins may be used alone, or two or more of these may be used as a blend.

The composition for a cover which forms the cover of the golf ball of the present invention preferably contains a thermoplastic polyurethane elastomer or an ionomer resin as its resin component. The ionomer resin may be preferably used in combination with a thermoplastic styrene elastomer. The amount of the polyurethane or ionomer resin in the resin component of the composition for a cover is preferably 50% by mass or more, more preferably 60% by mass or more, and still more preferably 70% by mass or more.

In addition to the resin component mentioned above, the composition for a cover may further contain any of pigmenting agents such as white pigments (e.g. titanium oxide), blue pigments, and red pigments, weighting agents such as zinc oxide, calcium carbonate, and barium sulfate, dispersants, antioxidants, ultraviolet absorbers, photostabilizers, fluorescent materials, and fluorescent brighteners, each in an amount which does not deteriorate the performance of the cover.

The amount of the white pigment (e.g. titanium oxide) is preferably 0.5 parts by mass or more, and more preferably 1 part by mass or more, whereas it is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, per 100 parts by mass of the resin component of the cover. Not less than 0.5 parts by mass of white pigment provides hiding properties to the cover. If the amount of the white pigment exceeds 10 parts by mass, the cover to be obtained may have reduced durability.

The slab hardness of the composition for a cover is preferably appropriately adjusted according to the desired performance of a golf ball. For example, in a distance-oriented golf ball for longer distance, the slab hardness of the composition for a cover is, in terms of shore D hardness, preferably 50 or higher, and more preferably 55 or higher, whereas it is preferably 80 or lower, and more preferably 70 or lower. The composition for a cover with a slab hardness of 50 or higher can provide a golf ball with a high launch angle and low spin on driver shots and iron shots and therefore a longer distance. Also, the composition for a cover with a slab hardness of 80 or lower can provide a golf ball with excellent durability. In a spin-oriented golf ball for better control, the slab hardness of the composition for a cover is, in terms of shore D hardness, preferably lower than 50, whereas it is preferably 20 or higher, and more preferably 25 or higher. The composition for a cover with a slab hardness of lower than 50 in terms of shore D hardness can provide a golf ball which has a longer distance on driver shots owing to the core in the present invention and also has higher spin rate enough to easily stop on a green on approach shots. Also, the composition with a slab hardness of 20 or higher can provide higher scratch resistance. In the case of having multiple cover layers, the slab hardnesses of the compositions for a cover forming each layer may be the same as or different from each other as long as each hardness is in the above range.

Examples of the method for forming the cover of the golf ball of the present invention include a method in which hollow shells are formed from the composition for a cover, the core is covered with the shells, and then the workpiece is compression-molded (preferably, hollow half shells are formed from the composition for a cover, the core is covered with the two half shells, and then the workpiece is compression-molded); and a method in which the composition for a cover is directly injection-molded onto the core.

In the case of forming the cover by compression-molding, half shells may be formed by either compression-molding or injection-molding. Here, suitable is compression-molding. The conditions for compression-molding the composition for a cover to form half shells are, for example, under a pressure of at least 1 MPa but not higher than 20 MPa at a molding temperature of at least −20° C. but not higher than +70° C. with respect to the flow beginning temperature of the composition for a cover. These molding conditions enable to form a half shell having a uniform thickness. Examples of the method for forming the cover using the half shells include a method in which the core is covered with two half shells and then the workpiece is compression-molded. The conditions for compression-molding the half shells to form a cover are, for example, under a molding pressure of at least 0.5 MPa but not higher than 25 MPa at a molding temperature of at least −20° C. but not higher than +70° C. with respect to the flow beginning temperature of the composition for a cover. These molding conditions enable to form a golf ball cover having a uniform cover thickness.

In the case of forming the cover by injection-molding the composition for a cover, an extrusion-pelletized composition for a cover may be used in injection-molding, or the base resin component and materials for a cover such as pigment may be dry-blended with each other and then directly injection-molded. Preferred upper and lower molds for forming the cover each have a semispherical cavity with pimples, some of which also serve as retractable hold pins. The formation of the cover by injection-molding may be performed as follows: the hold pins are stuck out; the core is put into the molds and is held by the pins; and then the composition for a cover is injected and cooled down to form a cover. For example, the composition for a cover heated to 200° C. to 250° C. is injected in 0.5 to 5 seconds into the molds clamped at a pressure of 9 to 15 MPa and is then cooled down for 10 to 60 seconds, and the molds are opened.

The cover typically has indentations called dimples, formed on its surface. The total number of dimples is preferably at least 200 but not more than 500. A total number of less than 200 dimples are less likely to exert their effects. Also, a total number of more than 500 dimples are less likely to exert their effects because they are so small in individual size. The shape (in a plan view) of each dimple to be formed is not particularly limited. Each of the following shapes: a circular shape; polygonal shapes such as substantially triangular shape, substantially tetragonal shape, substantially pentagonal shape, and substantially hexagonal shape; and other irregular shapes, may be used alone, or two or more of these may be used in combination.

The thickness of the cover is preferably 4.0 mm or smaller, more preferably 3.0 mm or smaller, and still more preferably 2.0 mm or smaller. The cover with a thickness of 4.0 mm or smaller can provide a golf ball with better resilience and shot feeling. The thickness of the cover is preferably 0.3 mm or greater, more preferably 0.5 mm or greater, still more preferably 0.8 mm or greater, and particularly preferably 1.0 mm or greater. The cover with a thickness of smaller than 0.3 mm may have reduced durability and abrasion resistance. In the case of having multiple cover layers, the total thickness of the cover layers is preferably in the above range.

After the golf ball body with the cover formed is taken out of the molds, it is preferably subjected to surface treatment such as burr removal, washing, and sandblasting, as appropriate. If desired, a paint layer and a mark may be formed. The thickness of the paint layer is not particularly limited, and is preferably 5 μm or greater, and more preferably 7 μm or greater, whereas it is preferably 50 μm or smaller, more preferably 40 μm or smaller, and still more preferably 30 μm or smaller. The paint layer with a thickness of smaller than 5 μm is likely to be worn and removed due to continuous use. If the thickness exceeds 50 μm, the effects of dimples may be reduced so that the flight performance of the golf ball may be reduced.

The golf ball of the present invention with a diameter of 40 to 45 mm preferably has a compression deformation (amount of shrinkage in the compression direction) measured from an initial load of 98 N to a final load of 1275 N of 2.0 mm or greater, more preferably 2.4 mm or greater, still more preferably 2.5 mm or greater, and most preferably 2.8 mm or greater, whereas the compression deformation is preferably 5.0 mm or smaller, and more preferably 4.5 mm or smaller. The golf ball with a compression deformation of 2.0 mm or greater has moderate hardness and gives good shot feeling. Conversely, the golf ball with a compression deformation of 5.0 mm or smaller has higher resilience.

The structure of the golf ball of the present invention is not particularly limited as long as it includes a spherical core having at least one layer, and a cover having at least one layer disposed to cover the spherical core. FIG. 5 is a partially cut-away cross-sectional view showing a golf ball 2 according to an embodiment of the present invention. The golf ball 2 includes a spherical core 4 and a cover 12 covering the spherical core 4. The cover has a large number of dimples 14 formed on its surface. The surface part of the golf ball 2 other than the dimples 14 is a land 16. The golf ball 2 has a paint layer and a mark layer outside the cover 12, but these layers are not illustrated in the figure.

The spherical core may have either a single-layer structure or a multi-layer structure. In the case of a multi-layer structure, the rubber composition for a core in the present invention may be used in any of the layers. Meanwhile, the cover has only to have one or more layers, and it may have either a single-layer structure or a multi-layer structure with at least two layers. Examples of the golf ball of the present invention include: a two-piece golf ball including a spherical core with a single-layer structure and a single-layer cover disposed so as to cover the spherical core; a multi-piece golf ball including a spherical core with a multi-layer structure and a single-layer cover disposed so as to cover the spherical core; a multi-piece golf ball including a spherical core with a single-layer structure and a cover having two or more layers and disposed so as to cover the spherical core; a multi-piece golf ball including a spherical core with a multi-layer structure and a cover having two or more layers and disposed so as to cover the spherical core; and a wound golf ball including a spherical core, a thread rubber layer provided around the spherical core, and a cover disposed so as to cover the thread rubber layer. The present invention can suitably be applied to any golf balls with the aforementioned structures.

EXAMPLES

The present invention will be described in detail referring to, but not limited to, examples.

Examples Corresponding to the First Aspect of the Present Invention [Methods for Evaluating Molded Product] (1) SANS Measurement

A plate-like sample (molded product) with a thickness of about 1 mm was swelled to equilibrium in deuterated toluene and attached to a sample holder in this state, and the sample was irradiated with a neutron beam at room temperature. Absolute scattering intensity curves obtained from measurements made at distances from the sample to a detector of 2.5 m and 10 m and from measurements using a focusing lens were combined by a least squares method. Regarding the combination of the three curves, the scattering intensity curve obtained from measurements made at a distance from the sample to the detector of 2.5 m was fixed, and then the scattering intensity curves obtained from measurements made at a distance of 10 m and from measurements using a focusing lens were shifted. Curve fitting with the Formulas 1 to 3 was performed on the resulting scattering intensity curve I_((q)), and the fitting parameter R_(g) was determined by a least squares method. A smaller R_(g) value indicates higher rebound resilience.

(SANS Device)

SANS: SANS measurement device provided with the SANS-J beamline of a JRR-3 research reactor belonging to Japan Atomic Energy Agency, an independent administrative institution

(Measurement Conditions)

Wavelength of neutron beam: 6.5 angstroms

Neutron flux intensity of neutron beam: 9.9×10⁷ neutrons/cm²/s

Distance from sample to detector: 2.5 m and 10 m (it is noted that measurement using a focusing lens was performed at a distance from the sample to the detector of 10 m for the purpose of obtaining information on the smaller angle side)

(Detector)

Two-dimensional detector (³He two-dimensional detector and two-dimensional photomultiplier+ZnS/⁶LiF detector)

(2) Metal Piece Collision Method

A 200-g-mass aluminum-made hollow cylinder was come into collision with a spherical sample (molded product) at a speed of 45 m/s. The speeds of the hollow cylinder before and after the collision and the speed of the sample after the collision were measured, and then the rebound resilience of the sample was determined. The obtained value of rebound resilience was expressed as an index value relative to the value in Example 1 defined as 100. A greater value indicates higher rebound resilience.

[Production of Molded Product]

The materials shown in Table 1 were kneaded using a Banbury kneader and a roll kneader, and then the kneaded material mixture was press-molded at 170° C. for 20 minutes to provide a molded product. The obtained molded product was evaluated as mentioned above.

[Method for Evaluating Golf Ball] (1) Coefficient of Restitution of Ball

A 198.4-g metal cylinder was come into collision with each golf ball at a speed of 40 m/sec. The speeds of the golf ball before and after the collision were measured, and the coefficient of restitution of each golf ball was calculated based on the speeds and the mass. For each golf ball, 12 balls were subjected to the measurement, and their average value was defined as the coefficient of restitution of the golf ball. In Table 1, the coefficient of restitution was given as the difference from the coefficient of restitution of the golf ball produced in Comparative Example 3.

[Preparation of Golf Ball] (1) Preparation of Core

A rubber composition with each formulation shown in Table 1 was kneaded using a kneading roll, and then heat-pressed in upper and lower molds each having a semispherical cavity at 170° C. for 20 minutes to form a spherical core with a diameter of 39.8 mm.

(2) Preparation of Cover

Next, materials for a cover with each formulation shown in Table 2 were extruded using a twin-screw kneading extruder to prepare a pelletized composition for a cover. The extrusion was performed at a screw diameter of 45 mm, a screw rotation rate of 200 rpm, and a screw L/D ratio of 35. The mixture was heated to 150° C. to 230° C. in the die of the extruder. The obtained composition for a cover was injection-molded so as to have a thickness of 1.5 mm onto the spherical core obtained as mentioned above. Thus, a golf ball including a spherical core and a cover covering the core was prepared.

With respect to the golf balls prepared, the coefficient of restitution was evaluated.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 1 2 3 Formulation BR730 100 100 100 100 100 100 100 100 100 (parts by mass) Sanceler SR 29 29 29 29 29 29 29 29 29 PERCUMYL D 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 TN 0.32 — — 0.32 — — 0.32 — — DPDS — 0.5 — — 0.5 — — 0.5 — Zinc octanoate 5 5 5 — — — — — — Zinc laurate — — — 5 5 5 — — — Method for SANS measurement 3.8 4.0 4.4 4.2 4.6 4.9 5.1 5.4 6.8 evaluating Radius of gyration R_(g)/nm rebound Metal piece collision 100 100 100 100 100 100 98 94 90 resilience Index value Coefficient of restitution of 0.019 0.016 0.011 0.013 0.009 0.007 0.006 0.004 0.000 ball BR730: high-cis polybutadiene (cis-1,4 bond content: 96% by mass, 1,2-vinyl bond content: 1.3% by mass, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weight distribution (Mw/Mn): 3) (JSR Corp.) Sanceler SR: zinc acrylate (coated with 10% by mass stearic acid) (SANSHIN CHEMICAL INDUSTRY CO., LTD.) PERCUMYL D: dicumyl peroxide (NOF CORP.) TN: 2-thionaphthol (Tokyo Chemical Industry Co., Ltd.) DPDS: diphenyl disulfide (SUMITOMO SEIKA CHEMICALS CO., LTD.) Zinc octanoate: product of Mitsuwa Chemicals Co., Ltd. (purity: 99% or higher) Zinc laurate: product of Mitsuwa Chemicals Co., Ltd. (purity: 99% or higher)

TABLE 2 Composition for cover Himilan 1605 50 Himilan 1706 50 Elastollan XNY97A — Titanium oxide  4 Unit: parts by mass Himilan 1605: sodium ion-neutralized ethylene/methacrylic acid copolymer ionomer resin (DUPONT-MITSUI POLYCHEMICALS CO., LTD.) Himilan 1706: zinc ion-neutralized ethylene/methacrylic acid copolymer ionomer resin (DUPONT-MITSUI POLYCHEMICALS CO., LTD.) Elastollan XNY97A: thermoplastic polyurethane elastomer (BASF Japan Ltd.)

As shown in Table 1, the R_(g) value of the molded product in each comparative example exceeded 5.0 nm, whereas the R_(g) value of the molded product containing zinc octanoate or zinc laurate in each example was 5.0 nm or less; thus, it is shown that the rebound resilience was high in each example. Moreover, the metal piece collision method demonstrated no difference in rebound resilience among the molded products in the examples; in contrast, the SANS measurement was able to demonstrate differences. This shows that the latter method enables to accurately measure a slight difference in rebound resilience between samples even though the difference is difficult to evaluate. In addition, a favorable correlation was observed between the R_(g) value of each molded product and the coefficient of restitution of each golf ball whose core was formed from the rubber composition.

Examples Corresponding to the Second Aspect of the Present Invention [Method for Evaluating Molded Spherical Core] (1) SANS Measurement

Plate-like samples (core-center sample, core-surface sample) each with a thickness of about 1 mm were swelled to equilibrium in deuterated toluene and attached to sample holders in this state, and the samples were irradiated with a neutron beam at room temperature. Absolute scattering intensity curves obtained from measurements made at distances from each sample to a detector of 2.5 m and 10 m and from measurements using a focusing lens were combined by a least squares method. Regarding the combination of the three curves, the scattering intensity curve obtained from measurements made at a distance from the sample to the detector of 2.5 m was fixed, and then the scattering intensity curves obtained from measurements made at a distance of 10 m and from measurements using a focusing lens were shifted. Curve fitting with the Formulas 1 to 5 was performed on the resulting scattering intensity curve I_((q)), and the fitting parameter G was determined by a least squares method. On the basis of the scattering length density difference σ (5.22×10¹⁰ (cm⁻²)) of polybutadiene (scatterer, swelled to equilibrium in deuterated toluene), the number N of scatterers (clusters each with a radius of gyration R_(g) of 1 nm to 100 μm) per unit volume was determined. The ratio (N value of core-surface sample)/(N value of core-center sample) was calculated based on the obtained N values.

(SANS Device)

SANS: SANS measurement device provided with the SANS-J beamline of a JRR-3 research reactor belonging to Japan Atomic Energy Agency, an independent administrative institution

(Measurement Conditions)

Wavelength of neutron beam: 6.5 angstroms

Neutron flux intensity of neutron beam: 9.9×10⁷ neutrons/cm²/s

Distance from sample to detector: 2.5 m and 10 m (it is noted that measurement using a focusing lens was performed at a distance from the sample to the detector of 10 m for the purpose of obtaining information on the smaller angle side)

(Detector)

Two-dimensional detector (³He two-dimensional detector and two-dimensional photomultiplier+ZnS/⁶LiF detector)

(2) Core Hardness

The JIS-C hardness was measured at a surface portion of a spherical core using a spring-type hardness meter Type C in conformity with JIS K6301, and the obtained value was defined as the surface hardness of the core. The spherical core was cut into two hemispheres, the JIS-C hardness was measured at the center of the cut surface, and the obtained value was defined as the central hardness of the core. Based on the obtained values, the difference: (surface hardness of core)−(central hardness of core) was calculated.

[Preparation of Molded Spherical Core]

The materials shown in Table 3 were kneaded using a Banbury kneader and a roll kneader, and then the kneaded material mixture was press-molded at 170° C. for 20 minutes to provide a molded spherical core for a golf ball.

A cylindrical test piece with a radius of 3 mm and a thickness of 1 mm was cut out of the center of the molded spherical core, and this piece was used as a core-center sample. Also, a cylindrical test piece with a radius of 3 mm and a thickness of 1 mm was cut out of the molded spherical core such that the circumferential part of the piece was in contact with the surface of the spherical core, and this piece was used as a core-surface sample.

The obtained core-center sample and core-surface sample were evaluated by the SANS measurement mentioned above, and the core hardness of the molded spherical core was also evaluated.

[Method for Evaluating Golf Ball] (1) Distance

A metal-head #W1 driver (XXIO S, loft: 11°) was attached to a swing robot (True Temper Sports, Inc.), and the driver was swung to hit the spherical core at a head speed of 40 m/sec. Then, the distance (distance (m) from the launch point to the stop point) was measured. Each golf ball was measured 10 times, and their average value was defined as the distance of the golf ball. The obtained value was expressed as an index value relative to the value of Comparative Example 4 defined as 100. A greater value indicates higher distance performance.

[Preparation of Golf Ball] (1) Preparation of Core

A rubber composition with each formulation shown in Table 3 was kneaded using a kneading roll, and then heat-pressed in upper and lower molds each having a semispherical cavity at 170° C. for 20 minutes to form a spherical core with a diameter of 39.8 mm.

(2) Preparation of Cover

Next, materials for a cover with each formulation shown in Table 4 were extruded using a twin-screw kneading extruder to prepare a pelletized composition for a cover. The extrusion was performed at a screw diameter of 45 mm, a screw rotation rate of 200 rpm, and a screw L/D ratio of 35. The mixture was heated to 150° C. to 230° C. in the die of the extruder. The obtained composition for a cover was injection-molded so as to have a thickness of 1.5 mm onto the spherical core obtained as mentioned above. Thus, a golf ball including a spherical core and a cover covering the core was prepared.

The distances of the golf balls prepared were evaluated.

TABLE 3 Example Comparative Example 7 8 9 10 11 12 13 14 4 5 6 7 Formulation BR730 100 100 100 100 100 100 100 100 100 100 100 100 (parts by mass) Sanceler SR 23 23 23 23 23 23 23 23 23 23 23 23 PERCUMYL D 1.5 1.8 1.8 0.3 1.5 1.8 1.8 0.3 1.5 1.8 1.8 0.3 TN 0.32 — — — 0.32 — — — 0.32 — — — DPDS — 0.5 — — — 0.5 — — — 0.5 — — 2,6-DCTP — — 0.36 — — — 0.36 — — — 0.36 — Zinc octanoate 5 5 5 5 — — — — — — — — Zinc laurate — — — — 5 5 5 5 — — — — Method for SANS measurement 4.1 3.8 3.4 3.1 3.4 3.2 3.0 2.7 1.9 1.8 1.7 1.6 evaluating N (core surface)/ hardness N (core center) difference Spring-type hardness 21 21 20 19 20 20 19 17 16 16 16 15 meter Type C Core surface − core center Index value of distance performance 106 104 101 94 104 102 99 92 100 98 97 91 BR730: high-cis polybutadiene (cis-1,4 bond content: 96% by mass, 1,2-vinyl bond content: 1.3% by mass, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weight distribution (Mw/Mn): 3) (JSR Corp.) Sanceler SR: zinc acrylate (coated with 10% by mass stearic acid) (SANSHIN CHEMICAL INDUSTRY CO., LTD.) PERCUMYL D: dicumyl peroxide (NOF CORP.) TN: 2-thionaphthol (Tokyo Chemical Industry Co., Ltd.) DPDS: diphenyl disulfide (SUMITOMO SEIKA CHEMICALS CO., LTD.) 2,6-DCTP: 2,6-dichlorothiophenol (Tokyo Chemical Industry Co., Ltd.) Zinc octanoate: product of Mitsuwa Chemicals Co., Ltd. (purity: 99% or higher) Zinc laurate: product of Mitsuwa Chemicals Co., Ltd. (purity: 99% or higher)

TABLE 4 Composition for cover Himilan 1605 50 Himilan 1706 50 Elastollan XNY97A — Titanium oxide  4 Unit: parts by mass Himilan 1605: sodium ion-neutralized ethylene/methacrylic acid copolymer ionomer resin (DUPONT-MITSUI POLYCHEMICALS CO., LTD.) Himilan 1706: zinc ion-neutralized ethylene/methacrylic acid copolymer ionomer resin (DUPONT-MITSUI POLYCHEMICALS CO., LTD.) Elastollan XNY97A: thermoplastic polyurethane elastomer (BASF Japan Ltd.)

As shown in Table 3, the ratio (N (core surface))/(N (core center)) of the molded spherical core in each comparative example was smaller than 2.0, whereas the ratio of the molded product containing zinc octanoate or zinc laurate in each example was 2.0 or higher; thus, it is shown that the molded products in the examples had sufficient hardness distribution. Moreover, the spring-type hardness meter demonstrated little difference in hardness among the molded products in the examples; in contrast, the SANS measurement demonstrated differences. This shows that the latter method enables to accurately measure a slight difference in hardness between samples even though the difference is difficult to evaluate. In addition, a favorable correlation was observed between the N value of each molded spherical core and the distance performance of each golf ball including the spherical core formed from the rubber composition. 

1. A rubber composition which has a radius of gyration R_(g) of 5.0 nm or less, the radius of gyration R_(g) being obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 3: $\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + l})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + l})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{79mu} {{{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\mspace{70mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)\mspace{70mu} {\left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right).}}} & \left( {{Formula}\mspace{14mu} 3} \right) \end{matrix}$
 2. The rubber composition according to claim 1, wherein the X-ray scattering measurement is small-angle X-ray scattering measurement and the neutron scattering measurement is small-angle neutron scattering measurement.
 3. The rubber composition according to claim 1, wherein the measurement is performed as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹.
 4. A golf ball, comprising: a spherical core having at least one layer; and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from the rubber composition according to claim
 1. 5. The golf ball according to claim 4, wherein at least one layer of the spherical core is formed from the rubber composition which contains: (a) a base rubber; (b) a co-crosslinker including at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof; (c) a cross-linking initiator; and (d) at least one of an acid and a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound.
 6. The golf ball according to claim 5, wherein the rubber composition contains 0.5 to 30 parts by mass of the at least one of an acid and a salt thereof (d) per 100 parts by mass of the base rubber (a).
 7. The golf ball according to claim 5, wherein the at least one of an acid and a salt thereof (d) each has 1 to 13 carbon atoms.
 8. The golf ball according to claim 5, wherein the at least one of an acid and a salt thereof (d) is at least one of a carboxylic acid and a salt thereof.
 9. The golf ball according to claim 8, wherein the at least one of a carboxylic acid and a salt thereof is at least one of a fatty acid and a fatty acid salt.
 10. The golf ball according to claim 9, wherein the at least one of a fatty acid and a fatty acid salt is a fatty acid zinc salt.
 11. The golf ball according to claim 5, wherein the rubber composition further contains (f) an organosulfur compound.
 12. The golf ball according to claim 11, wherein the organosulfur compound (f) is selected from thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 13. The golf ball according to claim 11, wherein the rubber composition contains 0.05 to 5 parts by mass of the organosulfur compound (f) per 100 parts by mass of the base rubber (a).
 14. The golf ball according to claim 5, wherein the rubber composition contains 15 to 50 parts by mass of the at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof (b) per 100 parts by mass of the base rubber (a).
 15. The golf ball according to claim 5, wherein the rubber composition contains the metal salt of a C3-C8 α,β-unsaturated carboxylic acid as the co-crosslinker (b).
 16. A golf ball, comprising: a spherical core having at least one layer; and a cover having at least one layer disposed to cover the spherical core, wherein at least one layer of the spherical core is formed from a rubber composition, and the golf ball satisfies the following relation: (the number N in an outermost portion of the rubber composition)/(the number N in an innermost portion of the rubber composition)≧2.0, where the number N is the number per unit volume of scatterers with a radius of gyration R_(g) of 1 nm to 100 μm, and the radius of gyration R_(g) is obtained by curve fitting of a scattering intensity curve I_((q)) obtained by X-ray scattering measurement or neutron scattering measurement, using the following Formulas 1 to 5: $\begin{matrix} {\mspace{70mu} {{q = \frac{4{{\pi sin}\left( {\theta/2} \right)}}{\lambda}}\left( {{\theta \text{:}\mspace{14mu} {Scattering}\mspace{14mu} {angle}},\mspace{14mu} {\lambda \text{:}\mspace{14mu} {Wavelength}\mspace{14mu} {of}\mspace{14mu} X\mspace{14mu} {rays}\mspace{14mu} {or}\mspace{14mu} {neutron}\mspace{14mu} {beam}}} \right)}} & \left( {{Formula}\mspace{14mu} 1} \right) \\ {I_{(q)} = {{\sum\limits_{i = 1}^{n}{\langle{{{P_{i}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{gi}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{fi}}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}} + {G_{i}{\exp\left( \frac{{- q^{2}}R_{g{({i + 1})}}^{2}}{3} \right)}}}\rangle}} + {P_{n + 1}\left\lbrack \left\{ {{{erf}\left( \frac{{qR}_{g{({n + 1})}}}{\sqrt{6}} \right)}^{3}/q} \right\} \right\rbrack}^{D_{f{({n + 1})}}}}} & \left( {{Formula}\mspace{14mu} 2} \right) \\ {\mspace{70mu} {{{erf}(z)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{z}{^{- t^{2}}{t}}}}}} & \left( {{Formula}\mspace{14mu} 3} \right) \\ {\mspace{70mu} {G_{i} = {N_{i}\left( {\sigma \; V_{i}} \right)}^{2}}} & \left( {{Formula}\mspace{14mu} 4} \right) \\ {\mspace{70mu} {{{V_{i} = {\frac{4}{3}{\pi\left( {\sqrt{\frac{5}{3}}R_{gi}} \right)}^{3}}}\mspace{70mu} \left( {P_{i},G_{i},R_{gi},{D_{fi}\text{:}\mspace{14mu} {Fitting}\mspace{14mu} {parameter}}} \right)\left( {N_{i}\text{:}\mspace{14mu} {Number}\mspace{14mu} {of}\mspace{14mu} {scatterers}\mspace{14mu} {per}\mspace{14mu} {unit}\mspace{14mu} {volume}\mspace{14mu} \left( {{pieces}\text{/}{cm}^{3}} \right)} \right)\; \left( {V_{i}\text{:}\mspace{14mu} {Volume}\mspace{14mu} {of}\mspace{14mu} {scatterer}\mspace{14mu} {having}\mspace{14mu} {radius}\mspace{14mu} {of}\mspace{14mu} {gyration}\mspace{14mu} R_{gi}} \right)\mspace{56mu} \left( {n\text{:}\mspace{14mu} {Integer}} \right)\mspace{56mu} \left( {q\text{:}\mspace{14mu} {Defined}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {same}\mspace{14mu} {manner}\mspace{14mu} {as}\mspace{14mu} {mentioned}\mspace{14mu} {above}} \right)}\mspace{56mu} \left( {z,{t\text{:}\mspace{14mu} {Any}\mspace{14mu} {positive}\mspace{14mu} {number}}} \right){\left( {\sigma \text{:}\mspace{14mu} {Electron}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {{electron} \cdot {cm}^{- 3}} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {matrix}\mspace{14mu} {material}\mspace{14mu} {or}\mspace{14mu} {scattering}\mspace{14mu} {length}\mspace{14mu} {density}\mspace{14mu} {difference}\mspace{14mu} \left( {cm}^{- 2} \right)\mspace{14mu} {between}\mspace{14mu} {scatterer}\mspace{14mu} {and}\mspace{14mu} {surrounding}\mspace{14mu} {deuterated}\mspace{14mu} {solvent}} \right).}}} & \left( {{Formula}\mspace{14mu} 5} \right) \end{matrix}$
 17. The golf ball according to claim 16, wherein the X-ray scattering measurement is small-angle X-ray scattering measurement and the neutron scattering measurement is small-angle neutron scattering measurement.
 18. The golf ball according to claim 16, wherein the measurement is performed as a function of q defined by Formula 1 in the range of not greater than 10 nm⁻¹.
 19. The golf ball according to claim 16, wherein the rubber composition contains: (a) a base rubber; (b) a co-crosslinker including at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof; (c) a cross-linking initiator; and (d) at least one of an acid and a salt thereof, and when the co-crosslinker (b) only includes the C3-C8 α,β-unsaturated carboxylic acid, further contains (e) a metal compound.
 20. The golf ball according to claim 19, wherein the rubber composition contains 0.5 to 30 parts by mass of the at least one of an acid and a salt thereof (d) per 100 parts by mass of the base rubber (a).
 21. The golf ball according to claim 19, wherein the at least one of an acid and a salt thereof (d) each has 1 to 13 carbon atoms.
 22. The golf ball according to claim 19, wherein the at least one of an acid and a salt thereof (d) is at least one of a carboxylic acid and a salt thereof.
 23. The golf ball according to claim 22, wherein the at least one of a carboxylic acid and a salt thereof is at least one of a fatty acid and a fatty acid salt.
 24. The golf ball according to claim 23, wherein the at least one of a fatty acid and a fatty acid salt is a fatty acid zinc salt.
 25. The golf ball according to claim 19, wherein the rubber composition further contains (f) an organosulfur compound.
 26. The golf ball according to claim 25, wherein the organosulfur compound (f) is selected from thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 27. The golf ball according to claim 25, wherein the rubber composition contains 0.05 to 5 parts by mass of the organosulfur compound (f) per 100 parts by mass of the base rubber (a).
 28. The golf ball according to claim 19, wherein the rubber composition contains 15 to 50 parts by mass of the at least one of a C3-C8 α,β-unsaturated carboxylic acid and a metal salt thereof (b) per 100 parts by mass of the base rubber (a).
 29. The golf ball according to claim 19, wherein the rubber composition contains the metal salt of a C3-C8 α,β-unsaturated carboxylic acid as the co-crosslinker (b). 